1. Field of the Invention
The present invention relates generally to communication devices, and more particularly to wireless communication devices that network their communications and must share the same limited communication resources (such as radio frequency spectrum). This sharing of resources leads to possible interference between communicating devices. The present invention provides for the individual wireless communication devices using the available communication resources in a manner that permits a mutual sharing of the resources by all of the wireless communication devices. This sharing of the communication resources is done in a way that tends to maximize the communication capabilities of the individual wireless communication devices under the constraint of not excessively reducing the communication capabilities of the other wireless communication devices sharing the same resources.
2. Description of the Related Art
Many wireless data link systems today have the ability to transmit at multiple power levels, at different data-rates, while choosing modulations using differing amounts of spectrum usage (i.e. bandwidth). The change in data-rate and bandwidth is typically accomplished by using variable amounts of Forward Error Correction (FEC) and/or changing the modulation-type. Since the wireless communication system will generally not be operating in isolation but typically operates in an environment of many other similar communicating wireless devices, it is desirable that the selection of the combination of transmit-power level, data-rate, and modulation-type, either achieve or approximate some optimum for the whole collection of communicating devices. A combination of transmit-power level, data-rate, and modulation-type will also be referred to hereafter as a “transmit mode” or simply as a “mode”.
Thus, a problem of choosing a true global optimum, i.e. the correct transmit mode for every wireless device, for an arbitrary collection of wireless devices would require extensive knowledge of all the states of the communication devices and the communication channel conditions between all communicating devices. This part of the problem itself would require a considerable data flow. If this data flow were to be carried across the same communication channels as provided by the wireless devices under discussion, these data flows could have severe adverse impact on the overall wireless communication network for its main desired purpose of passing data for users of the system; data passed through the system related to maintaining the communication network itself detracts from the network's main purpose. There is also the problem of computing the correct global optimum for the whole network and how this solution will be distributed. These considerations result in a nearly intractable problem for many kinds of wireless communication networks. Consequently, many wireless networks, the present invention included, as will be disclosed below in detail, take as an approximate solution the adjustment of the transmit mode for the individual communication links between pairs of nodes. This adjustment strives to achieve an acceptable level of communication performance while treating all the other communication in the wireless network as though they were merely uncoordinated interference to the desired communication. Treating all of the other wireless communication as interference is in contrast to calculating a globally optimum (or approximately globally optimum) solution based on exact information about all other wireless communication device behavior. The approach of treating other legitimate users as interference to the desired communication achieves the additional benefit of the wireless communication network responding appropriately when real sources of interference appear in the network. Communication between a pair of wireless communication devices will be referred to as a “communication link” or simply as a “link”. The approximate solution to resource sharing adjusts the transmit parameters for each link by “adapting” the parameters to achieve communication without excessive use of communication resources: hence the name “link adaptation”. A naïve approach to link adaptation would be for each pair of wireless communication devices to maximize communication resource usage for all the available parameters of its particular link. However, this would be counter-productive for many communication configurations as the available communication resources would not be shared; the mutual interference could actually eliminate the possibility of reliable communication. Consequently, link adaption, must be done in a way that results in a mixture of maximizing some aspects while minimizing other aspects to achieve the balance between the needs of the individual communication links and the performance of the complete collection of wireless communication devices taken as a whole.
The present invention provides a method of achieving this balance while also achieving other desirable characteristics not present simultaneously in other prior inventions. Some prior inventions achieve link adaption by increasing or decreasing a particular communication parameter in steps approximating a real-time negative-feedback control loop. This method requires almost constant two-way communication between the communicated devices. Although the present invention also requires two-way communication between the wireless devices, the present invention does this in a way that enables ad-hoc communication channels or massively asymmetrical amounts of communication between the nodes; nearly continuous communication that simulates a negative-feedback control system is therefore not necessary.
Prior inventions typically achieve link adaptation based on evaluating the performance of the communication taking place for one particular transmission mode—this transmit mode is typically the one presently in use and is the only one in use for that particular link. The present invention in contrast makes use of information derived from all communications that can be received by the receiving communication device from all of the transmissions made by the transmitting communication devices regardless of which transmit mode is being used and regardless of whether the communication is specifically directed at the receiving communication device. This supports ad-hoc type transmissions; multiple transmit modes might be in use for different communication purposes. The ability to use the information derived from several different transmit modes reduces the need for transmissions made specifically to achieve link adaptation.
Most link adaptation systems today either represent or can be put into a form that gives the solution space as a set of ‘states’ corresponding to all of the transmission modes that are possible. These adaptive algorithms then use signal-to-noise ratio plus possibly symbol error rate and other parameters to determine when to change from the current state to another state. Prior inventions typically limit the change to only those states that are connected to the state previously in use where the connections between the states are typically limited to a simple ordered list based on how robust a particular state is expected to be. Typically a heuristic algorithm decides when to change state, either more robust, or less robust from the predetermined simple ordered list. This simple ordered-state based approach does not work well in complex channels, such as interference channels or multipath channels. This is because the optimal or approximately optimal connections between the states change in a very complex manner as a function of the changing channel conditions; in other words, the states cannot remain a single simple ordered list. Additionally, many prior inventions determine the correct state based on first changing state then measuring the response to the change; this is undesirable if the transmissions are sporadic and infrequent. In contrast, the present invention does not use a simple ordered list of states. In principle each of the transmit modes are candidates for the new link adaptation solution starting from any other transmit mode. Although, the present invention could be viewed conceptually as a set of states with every possible state connected to every possible state, the present invention's innovation may be seen as determining and then changing immediately to the correct transmit mode without having to pass through intermediate ‘states’. This means there is no single intrinsic “robust” order to transmit modes since that order would depend on constantly changing communication channel conditions.
This lack of intrinsic ordering within the transmit modes is one of the strengths of the present invention allowing the link adaptation to be robust in adverse channel conditions. Even though the transmit modes don't have an intrinsic order, each of the parameters of power, data-rate, and modulation types that make up the transmit mode do have individual orders within the individual parameters. These internal orderings permit the present invention to infer many of the calculation results without explicit calculation of all possible transmit modes thereby resulting in a reduction in computational complexity.